ASoC: WM8985: Use the correct macro for the kcontrol defined

Since we are using custom get/put handlers
use SOC_ENUM_SINGLE_EXT_DECL instead of the original SOC_ENUM_SINGLE_DECL
macro.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>